The present invention relates to a high dielectric constant (hereinafter referred to simply as "high dielectric") material, a process for forming a high dielectric film containing tantalum, and to a semiconductor device using the film as a capacitor insulating film and the like.
In the present-day semiconductor devices, silicon nitride (Si.sub.3 N.sub.4) films are used as insulator films and capacitance insulating films. A capacitance insulating film is used, for example, as a capacitance for accumulating charge in a DRAM (dynamic random access memory). However, in an advanced ULSI (particularly DRAM), a material containing tantalum is studied as a material having a further higher specific dielectric constant than a silicon nitride, and its use in the place of a silicon nitride for a capacitance insulating film is considered. More specifically, for instance, a study on the use of Ta.sub.2 O.sub.5 is under way. In general, the capacitance C of a capacitance insulating film can be obtained according to the following equation: EQU C=.epsilon..times.S/d
where .epsilon. represents the specific dielectric constant, S represents the area, and d represents the thickness of the film. Because the area S of a capacitance insulating film is reduced inevitably with increasing density of the semiconductor device, the specific dielectric constant of the film must be increased to maintain the capacitance C at the same level. More specifically, in a bipolar transistor, for instance, the area of a capacitance insulating film increases as to account for a half of the entire area of the transistor element with decreasing area of the transistor element to one fifth of the initial size.
A tantalum oxide (Ta.sub.2 O.sub.5) which constitutes the capacitance insulating film yields a high specific dielectric constant in a range of from 20 to 30. For reference, the specific dielectric constant of Si.sub.3 N.sub.4 and SiO.sub.2 is in a range of from 6 to 7 and from 3.7 to 3.9, respectively. A Ta.sub.2 O.sub.5 film can be deposited by means of CVD process using gaseous TaCl.sub.5 and O.sub.2 as the starting materials.
As mentioned in the foregoing, a Ta.sub.2 O.sub.5 film has a high specific dielectric constant and is believed promising as a material for high dielectric capacitance insulating film. However, problems concerning leak current characteristics and isolation voltage are yet to be solved. As a means for overcoming these problems, there is proposed a process comprising depositing a Ta.sub.2 O.sub.5 film by means of CVD process on a body comprising a silicon substrate using gaseous TaCl.sub.5 and O.sub.2 as the starting materials, an d then applying an active oxygen annealing treatment (for instance, irradiating ultraviolet radiation in the presence of gaseous O.sub.2) to the as-deposited Ta.sub.2 O.sub.5 film. The inferior leak current characteristics and isolation voltage characteristics are believed attributable to the presence of Ta--O.sub.x voids that generate in the Ta.sub.2 O.sub.5 film during CVD due to the simultaneous drop out of tantalum atoms and surrounding oxygen atoms. Thus, by applying the active oxygen annealing treatment to the resulting Ta.sub.2 O.sub.5 film, silicon atoms which migrate from the silicon substrate and the oxygen atoms which diffuses into the Ta.sub.2 O.sub.5 film are believed to compensate for the deficient tantalum and oxygen atoms to fill the Ta--O.sub.x voids. Assumably, the leak current characteristics and the isolation voltage characteristics are improved according to this mechanism.
In add ition to the active oxygen annealing treatment described above, another means is proposed to further improve the leak current characteristics and the isolation voltage characteristics of a Ta.sub.2 O.sub.5 film. According to the process, a Ta.sub.2 O.sub.5 film is deposited by means of low pressure CVD (LPCVD) using gaseous Ta(OC.sub.2 H.sub.5).sub.5 and O.sub.2 as the starting materials, and then a high frequency oxygen plasma treatment is applied to the resulting Ta.sub.2 O.sub.5 film. By applying a high frequency oxygen plasma treatment to the Ta.sub.2 O.sub.5 film, carbon and hydrogen atoms in the Ta.sub.2 O.sub.5 film can be driven out of the film by diffusion, while filling the oxygen defects with oxygen atoms. The leak current characteristics and the isolation voltage characteristics are believed to be improved according to this mechanism.
In case of depositing a high dielectric film comprising Ta.sub.2 O.sub.5 by means of CVD, the use of a gaseous tantalum (Ta)-containing organometallic compound is increasing, because it can be handled with greater ease as compared with a gaseous TaCl.sub.5. However, in using a tantalum-containing organometallic compound as the starting gaseous material, carbon (C) is taken up by the amorphous high dielectric film during the deposition of the film, and the carbon contamination in a high dielectric film is found to be a serious problem to be overcome. The leak current characteristics are impaired by such a carbon contamination. It is possible to remove carbon from the high dielectric film to a certain extent, however, the efficiency of removal is still low.
As described in the foregoing, the high frequency oxygen plasma treatment above is found still insufficient for improving leak current characteristics and the isolation voltage characteristics. Accordingly, it is strongly desired to provide a high dielectric film superior to a Ta.sub.2 O.sub.5 film in terms of leak current characteristics and isolation voltage characteristics. The leak current characteristics and the isolation voltage characteristics of a high dielectric film can be effectively improved by increasing the thickness d of the high dielectric film. However, the capacitance C of the high dielectric film decreases with increasing thickness d. These conflicting requirements can be circumvented by selecting a material having a further higher specific dielectric constant .epsilon. for the high dielectric film.
Accordingly, an object of the present invention is to provide a tantalum-containing high dielectric material having a higher specific dielectric constant than that of a Ta.sub.2 O.sub.5.
Another object of the present invention is to provide a process for forming a tantalum-containing high dielectric film further improved in leak current characteristics and isolation voltage characteristics.
A still other object of the present invention is to provide a semiconductor device using the high dielectric film above.